Sedimentary records of human activity and natural environmental evolution in sensitive ecosystems: A case study of a coral nature reserve in Dongshan Bay and a mangrove forest nature reserve in Zhangjiang River estuary, Southeast China

Total organic carbon (TOC), total nitrogen (TN), delta C-13(org), delta N-15, and grain size were measured in two sediment cores from reserves adjacent to Dongshan Bay, China. The aims were to identify organic matter (OM) sources and to evaluate the combined impact of human activity and natural environmental processes over the last similar to 100 yr. In a coral reserve far from the estuary of Zhangjiang River, the average sediment accumulation rate (SAR) was 0.51 cm/yr from ca. 1839-2015, and the values of delta C-13(org) and C/N (atomic ratio of TOC to total N) ranged from -21.9 parts per thousand, to -23.7 parts per thousand, and from 5.7 to 8.1, respectively. In a mangrove forest reserve near the estuary, SAR averaged 0.49 cm/yr from ca. 1891-1985 and 2.41 cm/yr from ca. 1985-2015, and the delta C-13(org) and C/N values ranged from -22.7 parts per thousand, to -26.3 parts per thousand, and from 9.3 to 21.7, respectively. By combining a comparison plot of C/N and delta C-13(org) and a binary mixing model (based on delta C-13(org) and N-org/C-org, respectively), the major OM sources were assigned and their sedimentary contributions estimated. The sedimentary OM in the coral reserve had mainly an offshore marine authigenic signature with an average terrigenous contribution of <27%. In the mangrove forest reserve, the sedimentary OM was a mixture of terrestrial and marine sources, with an average terrigenous contribution of >56%. While input from the Zhangjiang River declined from the 1980s, urban development, deforestation and land reclamation contributed to an overall increase in the total mass accumulation rate (MAR). The coral was also severely damaged by the advent of poaching around the same time. These two factors collectively resulted in a rapid decrease in OM content and OC accumulation rate (CAR(org)) of the sediments until the establishment of the coral reserve in 1997. Changes in terrain and climate and the construction of water conservancy facilities were traditionally the primary factors responsible for changes in the sedimentary environment of the mangrove forest reserve. Since the 1980s, however, the expansion of the mangrove forest along with human activity has led to rapid increases in MAR and CAR(org) ; while, protective measures and a favorable climate have also promoted the growth of the mangrove forest. (C) 2018 Elsevier Ltd. All rights reserved.